The body includes a plurality of organs and systems that perform functions necessary for maintaining the health of a person. The circulatory system is one example of a system that includes the heart organ as its centerpiece. Other body systems include the respiratory system, digestive system, endocrine system, nervous system or the like. The organs of these systems provide a variety of physiological parameters useful for observing the normal and abnormal behaviors of the body. Observation of these parameters and recognition of potential normal and abnormal events through observation allows effective diagnosis or treatment of diseases, conditions or the like. The complexity of the various systems of the body provide multiple parameters that, when observed, provide insight regarding the onset of a condition or disease. Measuring each of these parameters and correctly identifying when measurements indicate a condition is difficult. Identifying a condition becomes even more difficult when some measurements indicate the onset or existence of a condition or disease while others do not.
One example of a body system is the circulatory system. The heart is the central organ for the circulatory system and includes an electromechanical system performing two major pumping functions. The left portions of the heart draw oxygenated blood from the lungs and pump it to the organs of the body to provide the organs with oxygen. The right portions of the heart draw deoxygenated blood from the organs and pump it into the lungs where the blood is oxygenated. The pumping functions are accomplished by contractions of the heart. An increase in the body's metabolic need for oxygen is satisfied primarily by a higher frequency of the contractions, i.e., a higher heart rate, along with changes in stroke volume.
Measurements of the various electrical and mechanical functions of the heart provide a variety of physiological parameters that can indicate the onset of a condition, for instance, heart failure, arrhythmia (fibrillation, tachycardia, bradycardia), ischemia, or the like. These physiological parameters include, for example, electrocardiac characteristics, heart sounds (e.g., S3 amplitude), DC impedance near the lungs, heart rate, respiration rate, weight, intracardiac pressure, blood flow, blood velocity, temperature, chemical presence and concentration or the like. At least some of these parameters may indicate the onset or change of a condition and thereby provide an alert that therapy or therapy adjustment is needed, such as defibrillation, change in pacing schema or the like. It is difficult, however, to determine when an event is beginning when only some measurements for these parameters indicate the onset of a condition.
In some examples, clinicians set measured parameter thresholds in implantable medical devices, such as pacemakers, defibrillators, cardiac resynchronization devices, or the like. Many clinicians adopt a conservative approach geared toward applying therapy even when therapy may not be needed. Therapy is thereby provided when at least one or more of the measurements for a parameter are above the set threshold—even when the measurements for other parameters indicate there is not an event. False positives, non-events that include measurements above at least some thresholds, thereby initiate treatment. In some circumstances, such as defibrillation shock therapy, the user of the implantable medical device receives painful and unnecessary treatment in response to such a false positive. The issues described above, with regard to cardiac therapy, such as setting conservative thresholds or the like, extend to other medical devices associated with the other organs and systems of the body.
Further, information about when many conditions begin may involve the left ventricle or left atrium. It is difficult to position sensors within the left side of the heart. The ability to precisely measure physiological parameters of the left side of the heart is thereby limited. Further still, because of the convoluted vasculature it is difficult to position electrodes for defibrillation and pacing therapy within the left side of the heart.
The present inventors have recognized that event detection systems and methods that address the above issues are needed. The present inventors have also recognized that what is further needed are implantable event detection systems capable of sensing multiple physiological parameters and providing increased specificity for delivery of therapy, such as in difficult to reach locations of the heart.